Quantum foam is a "material" at the smallest level of a universe's existence; the very foundation upon which all matter and energy rests. Spacetime bends around quantum foam at very slight increments, meaning that it likely would appear jittery or wavy if one were to observe it.
The exact size of quantum foam is unknown; some estimates put it in the yoctometers, while others may have it in Planck lengths or even smaller still.
It is thought that it may be linked to zero-point energy; a type of energy existing at the lowest possible state in a universe. Vacuum fluctuations involving virtual particles create and shape quantum foam using this energy. The exact mechanism by which they do this, and the amount of energy involved, is unknown.